For example, as described in Patent Literature 1, there is a conventional method of insulating a surface plate of a wire-cutting electric discharge machine in which legs of the surface plate fixing thereon a workpiece are formed of blocks with insulating properties so as to ensure insulation of the surface plate itself. Furthermore, there is another insulation method as described in Patent Literature 2, in which a surface-plate mounting table formed of a casting is provided below a surface plate and an insulation bushing is provided below the surface-plate mounting table, so as to insulate a workpiece mounting table itself that is constituted by the surface plate and the surface-plate mounting table.
Further, upon mounting a workpiece on a surface plate, the workpiece is required to be machined with micron accuracy, and therefore a surface of the surface plate is also required to have high accuracy. Accordingly, after the shipment of a machine, it is also required that the accuracy of the surface of the surface plate does not change with a large temperature increase and decrease within a transport container that transports the machine. For example, there is a case where a surface-plate mounting table that is a casting (a linear expansion coefficient 12), and a surface plate of stainless steel (a linear expansion coefficient 17) are insulated from each other by using ceramics (a linear expansion coefficient 7). In this case, a bimetal effect that is produced between materials with different linear expansion coefficients causes a problem of degrading the accuracy of a surface of the surface plate due to a temperature change during transport.
Normally, a surface plate is divided in such a manner as to suppress the bimetal effect (see Patent Literature 3).